1. Field of the Invention
The sensing apparatus and method of the present invention relate to the art of sensing liquids, vapors and gases. More particularly, the present invention relates to such an apparatus which is electrically actuated and which is responsive to changes in the electrical properties of the current saturated detection device used therein.
2. Description of the Prior Art
It is an increasingly important task in any industrial society to detect the presence of the liquid, vaporous and gaseous phases of many substances. For example, detection may be of consequence since the substance may be intrinsically hazardous due to its explosive, flammable, toxic or noxious character. Obviously, detection is doubly important when such substances enter confined areas where they are unwanted, such as living spaces, mines, bilges, storage tanks, trailers, aircraft and the like.
Detection may also be of import where, although the substance is not particularly perilous in itself, its presence is an indication of some undesirable condition. For example, a fire may be detected by its early products of combustion which, although toxic, may not be as hazardous as the fire itself. Similarly, the presence of a particular substance in the environment surrounding the sensing apparatus may indicate a leak in a supposedly tight system.
One of the many detection devices proposed is disclosed in U.S. Pat. No. 3,045,198, issued July 17, 1962, to Dolan, et al., of which applicant was a joint inventor. Applicant utilizes in his present invention a detection device fabricated in accordance with the disclosures contained in said patent.
In basic form, that patent discloses a detection device which includes a layer of resilient material which is secured to a rigid base member. The active element, a stratum of discrete, electrically conductive, adsorbent particles, adheres to the layer of resilient material, which serves to individually anchor each particle. A pair of spaced-apart electrodes which are in electrical contact with the stratum of conductive, adsorbent particles, completes the detection device. Under reference conditions, as upon exposure to pure atmospheric air, the detection device will normalize and develop a characteristic resistance which is a function of the resistance of the stratum of conductive, adsorbent particles located between the electrodes. However, when the detection device is exposed to the liquid, vapor or gas being sensed, it is found that its resistance changes, usually by increasing.
What is believed to occur is that, in accordance with known principles, minute quantities of the substance being sensed are adsorbed onto the surface of each adsorbent particle, thereby forming a uniform, monomolecular layer which coats the surface thereof. The force of adsorption, known as the Van der Walls' adsorption force, is so great that the layer of adsorbed substance will actually interpose itself between adjacent adsorbent particles which are normally in contact and separate them. As a result, conduction paths established during normalization, when the detection device was exposed to a reference environment, are disrupted, and the characteristic resistance of the detection device is changed, thereby signaling the presence of the sensed substance. As noted in the reference patent, the changed resistance of the detection device is correlated to the Van der Walls' "a" constant; and generally increases as the "a" constant increases.
Of course, the concentration of the sensed substance to which the detection device is exposed has a bearing on the response time of the device; but given sufficient time, even extremely low concentrations of the sensed substance will be noticeably sensed. Upon return of the detection device to the reference environment, the layer of adsorbate gradually dissipates, returning the adsorbent particles to their normal conductive contact, and thus returning the device to its characteristic, normalized resistance.
Through extensive experimental and other use of the detection device disclosed in the reference patent, applicant has discovered that while it was able to detect some substances, it was insensitive to others. Through further study and experimentation, applicant discovered an interesting correlation which gave an indication of being able to predict which substances were detectable by the detection device. Interestingly, Van der Waals' "a" constant seemed to be an indicator and further tests confirmed this hypothesis.
As a result of his experiments, applicant learned that the detection device of the reference patent, when used in accordance with the teachings disclosed therein, was generally sensitive to those substances having a Van der Walls' "a" constant which was greater than about 9, such as gasoline or diesel fuel, for example. However, its lack of sensitivity to certain other substances presented severe limitations on the usefulness of the referenced detection device, inasmuch as a host of common substances have a Van der Walls' "a" constant of about 9 or less. Thus, detection of carbon dioxide, carbon monoxide, propane, acetylene, natural gas and the like, all of which have an "a" constant of 9 or less, was impossible when using the prior art detection device in accordance with the disclosures contained in the reference patent.
Applicant has spent years of research effort in attempting to modify the prior art detection device so that it would be able to detect such substances, since he knew it would be an extremely useful and thus commercially valuable device if it could be so modified. For example, if it were able to detect carbon dioxide and carbon monoxide it could be fabricated as a component in a fire detector and thus be useful to save both life and property. If it were able to detect natural gas, for example, it could be fabricated as a leak detector for such equipment as natural gas pipelines or LNG (liquid natural gas) transport ships. Of course, many other applications for a workable detection device able to sense substances having a Van der Waals' "a" constant of less than 9 are readily apparent to those skilled in the art, and the uses mentioned are only by way of example.
In an effort to improve the prior art detection device and to make it sensitive to substances having a Van der Waals' "a" constant of 9 or less, applicant tried a multitude of approaches. Varying the adsorbent particle size, composition, and mixture did not work. Selection of different materials from which to fabricate the base member and resilient layer did not help. Changing the techniques by which the adsorbent particles were anchored to the resilient layer to thereby alter the depth and security with which each adsorbent particle was anchored also proved fruitless. Modifying the electrodes' composition and configuration proved to be a barren approach. No matter what was tried, applicant was unable to make a detection device that was sensitive to substances having a Van der Waals' "a" constant of less than about 9 when he followed the teachings of the reference patent.
However, through a fortuitous accident when applicant was measuring the current-carrying capabilities of the prior art detection device, he noticed that an anomaly occurred at certain voltage levels. That is, as the voltage across the detection device was increased, the current through the device also increased substantially in accordance with Ohm's Law. However, as the voltage was increased past certain levels, the current rose to a certain value and failed to substantially increase further, a result not predicted by Ohm's Law.
Fortunately, while the device was being operated in this current-saturated condition, i.e., small increases in voltage across the device failed to bring substantially the increases in current through the device as predicted by Ohm's Law; applicant decided to test it to see if it was still able to act as a sensor when operated in this condition. Much to his surprise, he found that the new apparatus was able to detect not only those substances having a Van der Waals' "a" constant of greater than about 9, but it was even able to detect those substances having an "a" constant of about 9 or less. A variety of liquids, vapors and gases were tested, and even helium, with an "a" constant of only 0.03412, was readily detectable. In each case, upon exposure to these substances, an easily detectable current change through the current-saturated detection device occurred that was superimposed on the milliampere order of base current flowing therethrough.
The discovery that the prior art detection device, when operated in a current-saturated condition, was sensitive to even those substances having a Van der Waals' "a" constant of about 9 or less was doubly surprising since one of the prime benefits of the prior art device was that it was a "cold" sensor. That is, because it operated at the ambient temperature and employed no hot elements, it could be safely used to detect even explosive or flammable substances. Its cold operation was the result of the fact that its resistance changed upon exposure to the sensed substance and thus only a few microamperes were needed to detect this resistance change.
Flooding the detection device with current, as applicant now specifies, so that it operates in a current-saturated condition, was not an intuitive step to take for two reasons. First, current saturation is unnecessary when detecting resistance changes in a resistance-type detection device. Secondly, it would be thought that a high current level through the device might raise the temperature of the device to a dangerously hot level or cause ionization of the device which would render it inoperative.
Although applicant is not certain of the exact explanation as to the operation of his current saturated sensing apparatus, he theorizes that when an electrical current of sufficient quantity is caused to pass through the conductive, adsorbent particles of the detection device employed therein, the particles are heated to a temperature just slightly above the temperature of the environment to which the sensing apparatus is exposed. Then, when the current saturated detection device is exposed to a substance with a Van der Waals' "a" constant of about 9 or less, the sensed substance is adsorbed on the adsorbent particles and the heat of adsorbtion thereby released slightly raises the temperature of the particles still further so that a readily detected current change through the device occurs. However, it should be noted that due to its unusual property of becoming current saturated by a mere milliampere order of base current, the detection device is still being operated as a "cold" type sensor, for such a minute current flow causes no substantial heating of the detection device as a whole.
It should be noted that there has been a change in the system of computing the Van der Waals' "a" constant since the referenced patent has issued, as is reflected in the current edition of the Handbook of Chemistry and Physics, published by the Chemical Rubber Company of Cleveland, Ohio. Applicant, in the present application, is using the current system.